In recent years, a lightweight eyeglass lens having a medium refractive index prepared from a glass having a relatively high refractive index n.sub.d of 1.60 to 1.61 but having a relatively small specific gravity of 2.5 to 2.6 has been commercially available. Such a lens has widely been made habitual use because lens is thinner and lighter than conventional lens having n.sub.d of 1.523 and provides improved sense of beauty and improved feeling of wearing. However, in the case that a fusion bonding type multi-focal lens is prepared using as a main portion a lightweight glass having a medium refractive index as described above, there is an inconvenience that the conventional glass for segment cannot be used. This is for the reasons that in the case of preparing a fusion bonding type multi-focal lens, it is generally required that the coefficient of thermal expansion of the segment be approximated to that of the main portion to prevent cracking after fusion bonding, and the softening point of the segment be set to a temperature slightly lower than the softening point of the main portion so as not to cause distortion of the main portion upon fusion bonding but the conventional glass for segment does not necessarily satisfy such requirements.
Therefore, a glass for segment having thermal properties suitable for a lightweight glass having a medium refractive index, namely, having a coefficient of thermal expansion of 82 to 95.times.10.sup.-7 and a softening point (Littleton Point) of 560.degree. to 640.degree. C., and having a high refractive index n.sub.d of 1.74 to 1.78 is demanded.
A glass which almost satisfies those conditions is, for example, SF 13 (coefficient of thermal expansion =80.times.10.sup.-7, softening point =604.degree. C., n.sub.d =1.74077) introduced in catalogue (1980 version) by Schott & Gen (West Germany). However, this glass involves disadvantages that not only the coefficient of thermal expansion is smaller than the desired value but also chromatic aberration is large due to its large dispersion of .nu..sub.d being 27.60. Accordingly, a large Abbe number is also the requirement necessary in the glass for segment.
In order to obtain a glass having a high refractive index, it is generally known to increase the proportion of components having high atomic valency of trivalent or more. As a result, however, the softening point of the glass increases and the coefficient of thermal expansion decreases. Conversely, the proportion of alkali metal oxides is increased in order to reduce the softening point of the glass and increase the coefficient of thermal expansion, but the refractive index decreases. Alkaline earth oxides impart middle properties between components of high atomic valency and alkali metal oxides. On the other hand, PbO decreases a softening point and reduces a coefficient of thermal expansion while it imparts a high refractive index.